Four major alkaloids are found in tobacco: nicotine, nornicotine, anabasine, and anatabine. Nicotine is the predominant alkaloid, usually accounting for more than 90% of the total alkaloids in commercial tobacco cultivars. Nicotine biosynthesis occurs predominantly in tobacco roots. Tobacco plants then transport nicotine through the vascular bundle to leaves where nicotine is then stored in the vacuoles.
A variety of factors affect tobacco alkaloid levels including genotype, environment, fertilization, and agronomic practices (for example, nicotine production is stimulated by topping, wounding, and herbivore damage). Low-alkaloid traits initially found in strains of Cuban cigar tobacco varieties were introduced into cigarette varieties through a series of backcrosses. Low-alkaloid tobacco germplasm was subsequently registered in the genetic background of cultivar Burley 21 (Legg et al., Crop Science, 10:212 (1970)). Genetic studies using the low alkaloid Burley 21 (LA BU21) lines indicated that two unlinked loci contribute to nicotine levels in the tobacco leaf. These two loci are referred to as Nic1 and Nic2. The nic1 and nic2 mutations in LA BU21 are semidominant. They show dose-dependent effects on nicotine levels, with the effects of nic1 about 2.4 times stronger than those of nic2. Molecular characterization of Nic2 locus has been reported. The nic2 mutation was shown to contain a deletion of a cluster of transcription factor genes from the ethylene responsive factor (ERF) family.
Reducing total alkaloid content in tobacco can have many benefits. It can increase the value of tobacco as a biomass resource. Increases in nicotinic alkaloid in tobacco plants may play an important role in protecting plants against insects and herbivores.
Consistent with alkaloids' role in insect defense, LA BU21 was reported to be extremely susceptible to insect damage (Legg et al., Crop Science, 10:212 (1970)). A further study comparing isogenic lines of flue-cured tobacco with low total alkaloids percentage (approximately 0.20%) with their “normal” recurring parents (total alkaloids 1.85 to 2.70%) reported that yield, grade index, total N, and reducing sugar content in the low alkaloid lines were lower than in the normal flue-cured cultivars (Chaplin and Weeks, Crop Science, 16(3):416-18 (1976)).
There is a need to identify novel genes that regulate tobacco nicotine levels, and to develop tobacco plants and products that contain altered nicotine levels (e.g., reduced nicotine) while maintaining (if not making superior) tobacco leaf quality.